Feedthrough assembly for cochlear prosthetic package

ABSTRACT

A cochlear prosthetic package having a cup-shaped titanium case with a ceramic plate; a plurality of hermetically-sealed feedthroughs are formed in the plate by sintering the plate with hollow metallic tubes located in plate holes and then sealing at least one end of each tube. An electrode cable connector is provided which allows removal and replacement of a case even in the presence of body fluids. A telemetry coil is contained in a metallic tube which extends out of the case; the coil-containing tube has its two ends hermetically-sealed to the case by ceramic insulating bushings.

This application is a division of application Ser. No. 350,574 filed May11, 1989, now U.S. Pat. No. 5,046,242 issued Sep. 10, 1991, which inturn is a continuation of application Ser. No. 402,227 filed Jul. 27,1982 and now abandoned.

This invention relates to implantable medical electronic devices such aspacemakers and cochlear prostheses, and more particularly to a cochlearprosthesis which must be hermetically sealed and have a telemetrycapability.

Implantable medical prostheses of many different types are now in commonuse throughout the world. These devices, such as pacemakers and bonegrowth stimulators, not only provide electrical stimulation but alsooften interact in two-way telemetry systems. Operation of a prosthesiscan be controlled by an externally transmitted signal, and theprosthesis itself can generate and transmit to the outside world asignal indicative of its operation or patient condition. There is agreat degree of cross-fertilization in the sense that techniquesdeveloped for one particular type of prosthesis are often eventuallyused in connection with others. A good example of this are thetechniques of hermetic sealing, originally developed for pacemakers butnow used for medical prosthesis in general.

Depending on the particular prosthetic device which is involved, thesolution of one problem may be more vexing than the solutions of others.But there is one type of device, a cochlear prosthesis, for which aconfluence of design criteria (some of which are competing) has severelylimited progress.

In its usual form, a cochlear prosthesis "system" consists of two parts.The first is an "electronics" package which is implanted in the mastoidbone behind the ear; a connector assembly, having perhaps 22 individualelectrodes extended to a cochlea, is removably attached to the package.The second part of the system consists of an externaltransmitter/receiver. The external unit not only functions in atelemetry capacity, but also serves to transfer power to the implantedunit.

The cochlear prosthetic package must be constructed in such a way thatonly bio-compatible materials are in contact with body tissues, a designcriterion common to implantable prosthetic devices in general.Similarly, the entire package should be hermetically sealed to preventthe ingress of body fluids which could have a damaging effect on theelectronic circuits in the package, and to prevent potentially harmfulsubstances which may be inside the package from contacting body fluids.Long life is assured by hermetic sealing, and it is known that apacemaker, for example, can operate for many years before a replacementis necessary, e.g., because of battery depletion.

The problem with a cochlear prosthesis is that entire replacement of thedevice is probably not feasible. The electrode assembly, once it hasbeen in place in a cochlea for several years, probably cannot beexplanted without damaging the cochlea itself. Thus the electrodeassembly itself must have a long life, e.g., fifty years. The packagecontaining the electronic circuits can be designed for long life, e.g.,as in pacemaker technology. However, in the unlikely event of a failure,it is more likely to be in the electronics package than the electrodeassembly. In addition, projected advances in technology will make itdesirable to be able to replace the electronics package with a moresophisticated unit. Thus it is necessary to be able to disconnect theelectronics package from the electrode assembly to enable replacementwith another package, and a connector arrangement is required.Permanent, hermetic connections (such as conventionally used inpacemakers) are unsuitable because disconnection and re-connection arenot possible.

The connector problem is much more severe in the case of a cochlearprosthesis than it is in the case of a pacemaker. A typical pacemakerrequires at most four electrodes to be connected to the internalelectronics via hermetically sealed electrical feedthroughs. Thepacemaker feedthroughs are much more widely separated than are those ofa cochlear prosthesis primarily because of the large number offeedthroughs in the latter, typically 10-22 in number. Each of the manyconnections between the hermetically-sealed electronics package and anindividual electrode in the electrode assembly must have low electricalresistance. Conversely, it has been found important that the resistancesbetween contacts be maintained as high as possible for long-term properoperation of the device. What further complicates matters is that notonly must the connector be capable of attaching the electrode assemblyto the electronics package several times without degrading theperformance, but each re-assembly must take place in an environmentwhere fluid ingress cannot be prevented; since the electrode assembly ispermanently implanted, every attachment of a new package to theelectrode assembly must take place in the patient's head.

It will be apparent to those skilled in the art that this type of"connector" problem is not limited to implantable medical prostheses.There are many fields of electronics where it is necessary to make asimilar high quality but difficult connection, such as in high pressure,high temperature, marine or chemically reactive environments. As willbecome apparent below, the connector aspects of the present inventionare equally applicable to such other electronic systems, as is thetechnique for making feedthroughs which will now be described.

The second major problem in the design and construction of a cochlearprosthetic package relates to the feedthroughs. The most pertinent priorart in this regard consists of techniques for making feedthroughs inpacemakers. A pacemaker is typically contained in a biocompatibletitanium case. In order to connect the electronics inside the package tothe electrodes, it is necessary to extend through the case up to fourconducting pins. This is usually accomplished by providing ceramicfeedthroughs. For each pin, a hole is provided in the titanium case anda ceramic bushing is placed in the hole. A conducting pin is extendedthrough a hole in the ceramic bushing to connect an external electrodeto the electronic circuit inside the case. An hermetic seal isestablished by utilizing brazing techniques--both at the case/ceramicinterface and the ceramic/pin interface. The ceramic bushings are notonly bio-compatible, but they also serve as insulators.

For a pacemaker, it is anticipated that more and more feedthroughs willbe required as the years go by. While most prior art pacemakers utilizeonly one or two feedthroughs, with the advent of dual chamber pacemakersit is apparent that four feedthroughs are desirable. Furthermore, asminiaturization techniques improve, it is expected that pacemakers willinclude many more sensing functions than they now have, and this will inturn require additional feedthroughs for connecting the electroniccircuits to sensor leads. But it is presently in the case of a cochlearprosthesis that the problems in fabricating the feedthroughs are mostsevere. In the illustrative embodiment of the invention, 22 feedthroughsare required. It is extremely time-consuming and costly to provide suchfeedthroughs using prior art techniques. If prior art techniques areused, not only must 22 individual feedthroughs be assembled, but abrazing operation at 22 sites is required--all in a very confined space.Because of the large number of hermetic brazes required to be performedat once, I have discovered that the brazing technique results in lowyields, leading to prohibitive manufacturing costs. It is clear that ifthe costs of making cochlear prostheses (and future pacemakers) are notto get out of hand, a different approach must be taken for implementingfeedthroughs.

Still another general problem relates to packaging of telemetry systems.The present invention does not concern itself with the electronicaspects of an implantable medical prosthesis, but rather with packagingof the telemetry coil. (Telemetry systems which are particularlyadvantageous for use in an implantable cochlear prosthesis are disclosedin Daly-Money applications Ser. Nos. 252,319 and 252,313, both filed onApr. 9, 1981, now respectively U.S. Pat. Nos. 4,408,608 and 4,533,988,and entitled respectively "Implantable Tissue-Stimulating Prosthesis"and "On-Chip CMOS Bridge Circuit", both of which applications are herebyincorporated by reference.) The package must be designed in such a waythat efficient transfer of power and information between the externaland internal coils is possible. The single coil in the implanted packagemay be used for either transmitting or receiving of information, orboth, as well as for receiving power from the external part of thesystem.

There are two standard prior art coil packaging designs. The first is toinclude one or more coils inside the same package which contains theelectronic circuits. This approach requires the package to be made of anonconducting material (i.e., not metal) to allow the efficient transferof power and data at useful frequencies. Achieving an hermetic seal insuch a case is difficult. Even if the package is non-metallic, but usesa metallic band to provide the hermetic sealing (e.g., involvingconventional brazing materials such as are used in the semiconductorindustry to provide hermetic sealing of integrated circuit packages),the seal itself will act as a short-circuited turn of wire and willdegrade the efficiency of power and information transfer. (Conventionalbrazing materials for joining metallic to ceramic components, e.g., inthe semiconductor industry, are often gold based, or use some othermetal, and have unproven biocompatibility. Brazing materials such ascommonly used in pacemakers for joining ceramic to titanium areacceptable.) The use of a metallic lid, even if it is on the side of thepackage facing away from the incoming radiation, also has a degradingeffect on the transmission efficiency. An even greater shortcoming ofthe coil-inside-the-package approach is that an obvious constraint isplaced on the size of the coil; in general, larger coils will allow moreefficient power and information transfer, but the package size islimited by implant requirements.

The second prior art approach involves using an external coil which isconnected to the electronic circuits inside the hermetically-sealedpackage via a pair of feedthroughs. There are two problems with thisapproach. The first is that it is not possible to hermetically isolatethe wire from the surrounding body fluids. While epoxy or Silasticmaterial may be placed around the coil, such coatings do not provide anhermetic seal and the ingress of body fluids can give rise to shortuseful life. The second problems relates to the large voltages which maybe generated. Typically, potential differences in the order of a voltmay be induced across the two ends of each individual turn in the coil.If there are 15 turns, as there are in the illustrative embodiment ofthe invention, a large voltage signal may be induced across the two endsof the coil as a result of the incoming signal from the outside world.Such a large potential across the feedthrough pins may lead tocorrosion, and may be biologically harmful.

It is a general object of this invention to provide feedthrough assemblyfor a prosthetic package including cochlear prosthesis or otherimplantable medical electronic device which overcomes the aforesaidproblems.

Briefly, in accordance with the principles of the invention, thefeedthroughs are made using an approach which is totally different fromthose of the prior art. A plate or carrier of unfired or "green" ceramicis formed with 22 small holes arranged around a larger central hole.Twenty 2-ended platinum tubes are placed axially into the smaller holes.Each tube has an outside diameter approximately the same as therespective hole so that a snug fit is obtained. The length of the tubeis about twice the thickness of the ceramic plate. I have discoveredthat a wall thickness of about one tenth the outside diameter of theplatinum tube gives good results. The platinum is 99.9% pure, or better.The tubes are placed on a flat, high alumina ceramic sheet so that oneend of the tubes is flush with one surface of the green ceramic plate.

The assembly is then placed in a furnace and the green ceramic issintered or fired in the usual way. As the ceramic sinters, it shrinks,typically in the order of 10-15% in all dimensions. The process ofshrinking causes pressure to be exerted around the platinum tubes, suchthat a platinum-to-ceramic reaction bond is formed between the tube andthe ceramic along the length of the tube in the ceramic, and around theentire circumference. The bond so formed results in an hermetic sealbetween the platinum and the ceramic.

The physical properties of the materials are important. First, a ceramicshould be used such that the firing temperature is about 0.9 times themelting point of platinum (i.e., about 1500 degrees C.). Other metalsand ceramics may be used, but this temperature ratio is important.Second, the platinum is a ductile material, and furthermore the processof heating and slow cooling anneals the platinum. Thus, in the coolingof the assembly, as the whole assembly shrinks further, the bond betweenthe platinum and ceramic is not broken by the platinum shrinking awayfrom the ceramic as a result of different coefficients of thermalexpansion, because the platinum is able to deform. Third, at thesintering temperature of the ceramic, the platinum material becomespartly plastic, and does not crack around the platinum tubes.

This technique is quite different from conventional platinum-to-ceramicreaction bonding techniques in several important ways. Conventionalbonding is usually between flat pieces of ceramic and platinum foil orsheet. I have developed this technique for a circular bond. In addition,it is extremely difficult to make a large number of platinum-to-ceramicbonds simultaneously using the conventional technique of externallyapplied pressure. Particularly important in this invention is the factthat independent pressure is produced in each hole around each platinumpart, and there is thus no theoretical limit to the number ofsimultaneous bonds possible; also, the dimensions may be extremely smallor quite large with good results. The prior art technique is often usedto join two pieces of ceramic together, and in order to make a singlesided bond, another refractory material is required to apply thepressure to the platinum. This is often graphite, but the use ofgraphite requires either a vacuum furnace or an inert atmosphere (e.g.,argon) to prevent the graphite from burning at the higher temperaturesinvolved.

Following sintering, at least one end of the tube is closed off, e.g.,by welding, to complete the hermetic seal. Alternatively, pre-formedplatinum parts with an end or ends already closed off may be used. It isimportant only that the portion of the platinum part in the ceramic isnot solid in cross section.

Despite the simplicity of the technique, it has been found thatconsistent perfect hermetic seals are achieved. (Thus far, the techniquehas been found to work only if feedthrough tubes, or tubular pre-formedparts, are used, rather than solid rods.) The net result is that aplurality of feedthroughs may be formed with a minimum of effort sincevery little individual feedthrough processing is required. A completelyhermetically sealed package may be obtained simply by brazing thecircumferential edge of the ceramic plate to an opening in an otherwisecompletely closed titanium case. Instead of requiring an individualbraze on each feedthrough, all that is required is a single brazing ofthe ceramic plate to the case. Close feedthrough spacings can beachieved because the only "work" required on each feedthrough is theclosing of at least one end of the respective tube. Even this can beeliminated by the use of pre-formed parts.

There remains the problem of connecting the electrode assembly to thecase in a manner which assures the electrical characteristics describedabove, as well as allowing occasional replacement of the case. In theillustrative embodiment of the invention, it is only the end of eachtube inside the case which is sealed. The open end of each offeedthrough, which extends away from the hermetically sealed case, isformed to have a concavity. The connector itself consists of a series ofplatinum wires (connected to the individual electrodes in the electrodeassembly), each of which terminates at an end shaped to mate with arespective formed feedthrough concavity. The wire contacts themselvesare embedded in a Silastic sheet (type 4120). This sheet is backed by atitanium cover, with another Silastic sheet (also type 4210) beinginterposed between the cover and the Silastic sheet which contains thewire contacts. A single screw extends up from the ceramic plate. (Thescrew has a special configuration, as will be described below.) Thisscrew passes through the two Silastic sheets and the titanium cover, andis secured by a nut. The force holding the connector together isprovided by a single centrally-located screw, but the force isdistributed over the connector surface by the elastic properties of theSilastic sheets, especially the sheet disposed against the cover. Aswill become apparent below, if the exposed surface of the ceramic plateis highly polished to a mirror finish, and the cover is held on withsufficient pressure, fluid is excluded in the connector which could forma conducting path between contacts.

As for the coil, the advantages of both the internal and externalapproaches are achieved, without the disadvantages of either. The coilis contained inside a metallic tube made of bio-compatible material. Thepreferred material is platinum, although titanium or stainless steel maybe suitable. The tube is attached via a pair of insulating ceramicbushings which are brazed to the titanium package. (The tube ends aresecured in the bushings, with an hermetic seal, in a manner comparableto the way the feedthroughs are made.) The metallic tube is thustopologically continuous with the inside of the package. Because theends of the tube are not electrically connected together (other thanthrough the tube), the tube does not form a shorted turn which otherwisewould make impossible the performance of the data and power link. Thereis no metal inside the coil, other than the tube which contains thecoil, which would otherwise similarly degrade the data and power linkefficiency. The coil size is not limited by the package, the tube can bebent to any desired shape, and a complete hermetic seal is achieved.

Further objects, features and advantages of the invention will becomeapparent upon consideration of the following detailed description inconjunction with the drawing, in which:

FIG. 1 is a top view of the package of the illustrative embodiment ofthe invention (with the details of the electrode assembly being omittedsince such assemblies are well known in the art);

FIG. 2 is a side view of the package of FIG. 1;

FIG. 3 is an exploded perspective view showing the several componentparts of the package of FIG. 1;

FIG. 3A is a sectional view showing how ceramic plate 32 is secured tolid 44;

FIG. 4 is a detailed view showing the manner in which a connectorcontact is made with each of the feedthroughs;

FIG. 5A is a detailed view illustrating the shape of the screw whichextends through the connector, and FIG. 5B is an alternative embodimentof the screw; and

FIGS. 6A, 6B and 6C illustrate the manner in which the feedthroughs arefabricated, with FIG. 6A depicting the start of the feedthroughfabrication, and FIGS. 6B and 6C showing successive steps performed onan individual feedthrough in the overall method.

The package illustrated in FIGS. 1 and 2 includes a titanium case 10,and a titanium connector cover 18. The cover is coupled to the case by atitanium screw 42 and a titanium nut 20. Cable 16, extended to theelectrode assembly (not shown), exits the cover 18 as shown. A pair ofceramic insulating bushings 14 are brazed to the titanium case, as shownat 14a, in a conventional manner.

Platinum tube 12 has two ends inserted into the holes which extendthrough the ceramic bushings, and the ends of the tube are hermeticallysealed to the bushings as will be described below. An important featureof the design is that the two ends of the tube are not electricallyconnected so that the tube does not comprise a shorted turn which wouldotherwise absorb radiated power. In the illustrative embodiment of theinvention, the coil (not shown) comprises 15 turns, each turn passingthrough the tube and the inside of the case from one bushing to theother. The two ends of the 15-turn coil are connected to the electroniccircuits (not shown) inside the case.

An alternative embodiment which has been found to be useful is to use asingle-turn coil which is coupled into a small transformer wound on aferrite toroid, where the number of turns on the secondary may beadjusted to give the required voltage, and may be optimized for besttransfer efficiency. In one embodiment, the single-turn coil can be asingle turn of insulated copper wire contained within the platinum tube.Alternatively, multistranded platinum wire may be welded to the platinumtube coming through the ceramic feedthroughs brazed into the titaniumcase. That is, instead of the tube being continuous from one ceramicbushing to the other, only short pieces of tube are used, and solid orstranded wire is hermetically welded to the tubes to complete the coil.This technique has the advantages that the whole assembly is morerobust, since the receiving coil is solid wire, and can be easily bent(as opposed to a tube which is subject to kinking and fracture onbending). In addition, there are some advantages to be gained in theelectrical performance by using a single-turn receiver coil.

As mentioned above, the details of the electronic circuits are notimportant for an understanding of the present invention. The entireassembly may be encased in Silastic (not shown), to insure that no sharpedges are exposed, and to cushion and protect the package once it isimplanted in the body. To remove the case from the connector theSilastic can be cut. When a new case is then attached to the connector,the new case can be encased in Silastic before reimplantation. The onlydifference between the Silastic coatings around the overall initialimplant and around a subsequent implant is that the coating is notcontinuous in the latter where the connector is secured to the case.This is of little moment because the Silastic coating does not functionas an hermetic seal in the first place.

The overall assembly can be best appreciated by considering the explodedview of FIG. 3. A general description of the arrangement of parts willbe described, followed by a more detailed consideration of the salientfeatures of the invention.

Titanium case 10 is cup-shaped and can be made by machining a solid rodor by sheet metal forming. Two holes are drilled in the side of the cupfor the coil insulating ceramic bushings 14. A platinum (in thepreferred embodiment) tube 12 is attached to the bushings such that anhermetic seal is formed. The bushings are attached to the cup by usingconventional ceramic-to-titanium brazing techniques (see 14a in FIG. 2),techniques which are well known to manufacturers of pacemakers.

Ceramic plate 32 (illustratively circular) contains multiple conductingfeedthroughs 34. The manner of securing the feedthroughs in the ceramicplate and achieving an hermetic seal, as well as the detailed shape ofthe feedthroughs, will be described below. Titanium screw 42 is insertedthrough hole 32a of the ceramic plate, and the head of the screw isbrazed to the underside of plate 32, around hole 32a, so that anhermetic seal is formed. At the time that the screw is attached to theceramic plate containing the feedthroughs, the plate is brazed totitanium lid 44 around its circumference, as shown by the numeral 68 inFIG. 3A. It should be observed that screw 42 has a position which isgenerally central to the positions of the feedthroughs.

The electronic components are then assembled on lid 44 with electricalconnections being made to all of the feedthroughs. The coil is woundwithin tube 12 as described above and the two ends are connected to theelectronic circuits. Lid 44 is then attached to the wall of cup 10 witha circumferential weld using conventional TIG titanium weldingtechniques.

The electronic circuits inside the case may be mounted on a flexibleprinted circuit board which is soldered to the feedthroughs and the twoends of the coil. Alternatively, if the number of electronic componentsis small, they may be assembled inside the case using point-to-pointwiring. For example, an integrated circuit chip carrier may be attachedby glue to the end of the titanium screw, with wires being run betweenthe feedthroughs and the connections on the chip carrier. Smallcomponents may be soldered in place onto the chip carrier, as may theend connections of the coil threaded through the tube.

The case thus completed is one part of the cochlear prosthesis package,the internal replaceable part. It should be noted that the unit ishermetically sealed. Although it might appear from the description thusfar that the feedthroughs open the inside of the case to the outside,reference to FIG. 4, to be described in detail below, shows that thebottom of each feedthrough tube is closed (prior to lid 44 being weldedto cup 10).

The remaining elements in FIG. 3 constitute the connector for connectingthe electrodes to the feedthroughs, the connector being permanentlyattached to the electrodes and ideally never requiring replacement.Silastic sheet 36 contains 22 preformed platinum parts 30 which matewith the feedthroughs on the ceramic plate 32. The preformed parts inFIG. 3 are shown as terminating in spheres, although in FIG. 4 theirends are shown as having a conical shape; these are only two of thepossible shapes which may be used. FIG. 4, which depicts theconstruction of the elements associated with Silastic sheet 36, will bedescribed in detail below. A wire 26 is connected to each connectorelement 30, and all of the wires are extended to electrode cable 16(which does not form part of this invention). See, generally,"Development of Multichannel Electrodes For An Auditory Prosthesis",Report on Progress, Sep. 1, 1980 through Nov. 30, 1980, NIH Contract No.N01-NS-0-2337, by Merzenich et al.

Silastic sheet 24 is placed against the inside flat surface of a rigidtitanium cover 18. Silastic sheet 36 is positioned against Silasticsheet 24. Actually, Silastic sheet 36 is molded inside cover 18. Thecover is inverted and Silastic sheet 24 is placed in it. The 22connector parts are then held in place while Silastic material is pouredon top of sheet 24 in order that sheet 36 be formed to hold theconnector parts. It should be noted that sheet 24 and cover 18 have arespective cut-out and hole 24a, 18a for allowing cable 16 to passthrough the cover. Holes 36a, 24b and 18b allow screw 42 to pass throughthem during final assembly.

During the final assembly step, the connector is placed on top ofceramic plate 32, with each individual connector part in the connectorbeing seated in a respective one of the feedthroughs as will bedescribed in connection with FIG. 4. Screw 42 extends up through theconnector and the two parts are held in compression against each otherby tightening titanium nut 20 on the screw. The head on nut 20 sits inthe depression which surrounds hole 18b in the cover.

FIG. 5A depicts the manner in which screw 42 is attached to theunderside of ceramic plate 32. The screw is shaped so that it has anundercut 42a in its head. The brazing of the screw to the ceramic plateis shown at 50, and because of the undercut the brazing takes place on athin ring around the edge of the screw head. The reason for insuringthat the brazing takes place only along a thin ring is that were thebrazing to be over the entire flat undersurface of the screw head, thebrazed joint would be subject to strong shear forces during coolingafter the brazing, and these shear forces would tend to break the bond.By insuring that the braze is only over a relatively small area, thiseffect is reduced. Undercut 42a, and the fact that the diameter of hole32a is larger than the diameter of the screw, provide minimal contactbetween the screw and the ceramic feedthrough carrier.

An alternative embodiment for attaching the screw to the ceramic plateis shown in FIG. 5B. In this technique, another platinum tube 61 (oflarger diameter and longer than the tubes used for the feedthroughs) isjoined to the ceramic plate 60, in the same way as the platinum tubesfor the connector contacts, that is, by reaction bonding. Afterattachment to the ceramic, the exposed end of the tube is flared (orthis may be preformed prior to assembly) and a screw with a head ofmatching shape 62 is inserted into the platinum tube. The tube is thenfolded over the head of the screw and joined, e.g., by welding orbrazing as shown by the numeral 63. This technique has the advantagethat a metal-to-metal (i.e., platinum tube-to-screw) bond is required,and may thus be done in a variety of ways utilizing a variety ofmaterials for the screw, e.g., titanium, platinum or biocompatiblestainless steel. In addition, by appropriate choice of the shape of thescrew head, all the rotational forces on the screw during tightening ofthe connector are not borne by the bond between the tube and the screw,but by the geometric arrangement of the screw head and tube. Forexample, if the screw head were hexagonal instead of circular, then thedeformation of the platinum tube over the screw head would tend tostrongly hold the screw in place. Thus the major function of the bondbetween the platinum tube and the screw is to provide an hermetic seal.

The feedthrough construction is illustrated in FIGS. 6A-6C. Greenceramic 32, formed to shape but not yet sintered, is sized so thatfollowing sintering and the resultant shrinking, the plate will fit inlid 44 (FIG. 3) as desired. Holes in the required positions for thefeedthroughs are drilled in the green ceramic plate, and a further hole32a is drilled to allow screw 42 (FIG. 3) to pass through it. Platinumtubes 34 are then placed in respective holes in the plate. The length ofeach tube, relative to the thickness of the plate, is shown in FIG. 6B.The tubes fit snugly in the holes in the plate, and they are placed sothat a relatively large length protrudes from the upper surface of theplate (the surface which is shown as the undersurface in FIG. 3), andonly a small length protrudes from the bottom. As seen most clearly inFIG. 6B, the thickness of the wall of each of tubes 34 is aboutone-tenth the outside diameter of the tube.

The green ceramic plate containing the platinum tubes in position isthen placed on a flat surface within an oven and the oven is operated atthe appropriate temperature and for the appropriate time necessary tosinter the ceramic plate. The sintering is no different from that of theprior art. As the ceramic sinters it shrinks, and thus shrinks aroundeach of the platinum tubes. With the pressures involved due to theshrinkage, and the temperatures of sintering, a platinum reaction bondis formed between the ceramic and each platinum tube along its entirelength in the ceramic around its circumference, to form an hermetic sealbetween the ceramic and the platinum tube. The process for forming somany hermetic seals in so tight a space is amazingly simple andcost-effective. The physics involved in the reaction bonding process isnot completely understood. However, there is no question that theresults are reproducible with every seal being hermetic. Aftersintering, the outer circumference of the plate may be ground to thecorrect dimensions, if necessary.

The general technique of reaction bonding metals to ceramics or glassesis known, and is used, for example, in the manufacture ofhigh-temperature platinum thermocouples. However, prior art techniquesrequire the application of external pressures under high temperature toachieve the bond. In the practice of the present invention, it is theforces associated with the shrinkage of the ceramic during sinteringthat achieves the bond. It is also known in the prior art to use ceramicwhich is initially sintered. The prior art techniques are describedgenerally in Klomp, "Bonding of Metals to Ceramics and Glasses", CeramicBulletin, Volume 51, No. 9 (1972).

The ceramic plate is initially formed so that the bottom end of eachfeedthrough hole through the plate has a conical shape as shown by thenumeral 32b in FIG. 6B. FIG. 6C depicts, in enlarged form, an individualplatinum tube within the ceramic plate after the plate is fired andafter subsequent processing. The bottom end of each tube is preferablyformed to a concave shape, as shown by the numeral 34c in FIG. 6C, toprovide a better contact area for the respective connector part. Thebottom surface of the ceramic plate is then lapped to provide ahigh-quality "mirror" surface finish, for a reason to be describedbelow. Finally, the upper end of each platinum tube (which is the lowerend in FIG. 3) is closed off by using a standard welding technique, inorder to complete the hermetic seal. The upper end is pinched, as shownby the numeral 34a in FIG. 6C, following which the tip is welded asshown by the numeral 34b.

Instead of forming the connector parts after firing, it is possible touse preformed platinum parts with closed ends. The region of theconnector part which passes through the ceramic should be tubular.Reaction bonding has been found to take place when the connector part isa hollow tube in the region where the bonding is desired.

A similar technique is utilized for securing the ends of tube 12 withinceramic carrier bushings 14 (FIG. 3), since hermetic seals are alsorequired for the tube ends. Ceramic bushings are made of green cermic,each having a central hole such that one end of hollow tube 12 fitssnugly in the hole. (As in the case of ceramic plate 32, if there is alarge degree of shrinking, the part being secured within a hole in aceramic piece need not even fit snugly around the inserted tubularpart.) The entire tube and the two bushings at its ends are then placedin an oven so that the ceramic bushings sinter. The bushings shrink andonce again hermetic bonds are formed between each bushing and theplatinum tube which it surrounds. A further advantage of this approachis that in the process the tube is annealed so that it may be easily andsafely bent to any desired shape during implantation. After the bushingsare thus attached to the ends of the tube, the bushings are attached tothe titanium case 10 using any conventional brazing technique employedin fabricating pacemaker feedthroughs. During this process, care must betaken that none of the braze alloy be allowed to form an electricalconnection between the metal case and the coil-enclosing tube. Animportant feature of the construction is that while the inside of thetube and the inside of the case are topologically continuous, they arenot electrically connected so that the tube does not form a shorted turnwhich would otherwise absorb electromagnetic radiation to a significantdegree.

An enlarged view of the manner in which a connector part makes contactwith a feedthrough is shown in FIG. 4. Ceramic plate 32 is shown with asingle feedthrough 34, the upper end of the feedthrough having aconcavity 34c. At the bottom of the connector a platinum wire 30terminates in a conical head 30a (although a spherical termination, asshown in FIG. 3, as well as other shapes, also suffice). An enlargedhead is advantageous in that it contributes to better seating of the pinin the feedthrough, as well as providing a larger contact area. Pin 30is held in molded Silastic sheet 36, this sheet bearing against Silasticsheet 24 which is adjacent to titanium cover 18. Pin 30 is connected toa wire 26, the wire in turn being extended through the electrode cableto a particular electrode. One end of wire 26 is resistance welded topin 30, as shown by the numeral 28. Each of pins 30 is in realitynothing more than a short segment of wire, one of whose ends is formedinto a desired shape. The wire has a diameter of 0.005 inches. The wireis too thick to be extended directly through the electrode cable, andwire 26 has a diameter of only 0.001 inches. It is for this reason thatthe two of them must be welded together within the connector; the wirewhich contacts the feedthrough is too thick for the electrode cable, andthe wire in the electrode cable is too thin to establish sufficientcontact with the feedthrough.

The upper surface of ceramic plate 32 is highly polished, as describedabove. Even though only a single screw is utilized for establishing theconnection, due to the provision of Silastic sheet 24, Silastic sheet 36is held against the ceramic plate with an even and consistent pressure.(It is metallic cover 18, of course, which distributes the screw forceto the backing Silastic sheet 24.) Because of the uniform pressurethroughout the interface between Silastic sheet 36 and ceramic plate 32(resulting from the resiliency of the Silastic sheets), and because ofthe high polish on the surface of the latter, any fluid between the twosurfaces is squeezed out into the empty space around head 30a of theconnector part, the empty space being shown by the numeral 38 in FIG. 4.The fact that fluid may surround a connector part is of no moment; whatwould be a problem would be the existence of fluid between adjacentfeedthroughs or connector parts and it is for this reason that fluid isexcluded at the interface of the ceramic plate and Silastic sheet 36.The high polish of the ceramic plate prevents fluid from being trappedin microscopic depressions in the surface so that a very highinter-electrode resistance may be maintained.

It should be noted that the arrangement of connection points is suchthat the connector is self-locating, i.e., it can only be assembled inone way. Proper placement of the connector, with each of the 22connector parts fitting in a respective feedthrough, can be sensedduring assembly, and only when proper seating is achieved should nut 20(FIG. 3) be tightened on screw 42.

Referring to FIGS. 1-3, it will be seen that tube 12 extends to one sideof the circumference of the case. Because the case is outside thecircumference of the coil, except for the short segment of the each turnwhich goes through the case between the ceramic bushings, the metal ofthe case does not significantly degrade the performance of the data andpower link. It is because the two ends of the tube are not electricalllyconnected together that the tube does not act as a shorted turn whichwould otherwise absorb most of the radiated power. The tube simply actsas an open circuit turn, with potentials in the order of a volt beingdeveloped across its two ends.

Other orientations of the tube are possible. For example, the tube mightbe bent so that its plane is parallel with the plane of the case withthe case contained within the tube. With such an orientation,transmission is still possible provided that the tube, with the coilinside, is between the case and the external transmitter/receiver. Ofcourse, were the case to be interposed between the tube and the externaldevice, the case would absorb practically all of the radiated power.

The use of a soft or ductile metal allows the tube to be bent to conformto the shape of the cavity into which the package is to be placed. Thefact that there is little conductive metal within the coil (e.g., thewall of the tube) is advantageous in that any metal placed inside thecoil absorbs power. Additional metal inside the coil comprises a shortsegment of the case wall between two ceramic bushings. Even this can beavoided if the case itself is made entirely of ceramic. The disadvantageof using ceramic material, however, is that a thicker wall thicknesswould be required, thus increasing the volume of the case.

It should also be appreciated that the shape of the tube need not becircular; any shape required by the implant and the anatomical site ofimplantation may be used. The diameter of the coil and its enclosingtube is not determined by the diameter of the case containing theelectronic circuits, thus allowing flexibility in design and a reductionof the total volume of the implant. Another advantage of the use of thetube is that it may serve as a convenient anchoring point for theimplant, either by using sutures or by including a fibrous mesh acrossthe tube, should there be no other convenient way to anchor the implant.

Referring to FIG. 1, it will be seen that the unit is symmetrical arounda vertical plane through the center. The advantage of this is that thepackage is not "handed", eliminating the need to make two differentversions of the package for either side of the head.

The connector configuration satisfies all of the requirements for acochlear prosthesis. The firm seating of the connected parts in thefeedthroughs and the large contact areas insure that there is lowelectrical resistance in series with each electrode lead. Currentleakage paths between connector points have high electrical resistancedue to the usage of the Silastic/polished-ceramic interface. Theconnector may be disconnected and connected to another case a smallnumber of times without any degradation in performance, even thoughreconnection in a fluid-surrounded body environment does not allow fluidingress to be prevented.

Needless to say, one of the most important characteristics of theoverall unit is that only well-proven bio-compatible materials are used,including titanium, platinum, ceramic, and medical-grade Silastic.

Although the invention has been described with reference to a particularembodiment, it is to be understood that this embodiment is merelyillustrative of the application of the principles of the invention. Forexample, the feedthrough pieces (which can be pre-formed) need not becircular in cross-section; an elliptical or even a non-uniform shape maybe acceptable. Similarly, instead of platinum, other noble metals may beacceptable. Nor need the feedthroughs function in an electricalcapacity. By utilizing tubes both of whose ends are not sealed,fail-safe communication can be had to a hostile environment, e.g., tosample chemical reactions such as in a blast furnace or in plasticsmanufacturing, to introduce reagents, or to perform a sensing function.The connector may be used for a cable-to-cable coupling rather than acable-to-case coupling. In those cases where a telemetry capability isrequired, the package might be provided with only one tube bushing,solid or stranded wire being welded to the tube and functioning as an"aerial" with a free end. Thus it is to be understood that numerousmodifications may be made in the illustrative embodiment of theinvention and other arrangements may be devised without departing fromthe spirit and scope of the invention.

I claim:
 1. A feedthrough assembly for a prosthetic implant, comprising:a carrier of a sintered ceramic material and having at least one holetherein, and at least one 2-ended hollow tubular feedthrough element ofa platinum material extending axially through said at least one hole,said ceramic material having a sintering temperature of about 85-90% ofthe melting point of said platinum material and having the property ofbeing solid both in its green state and its sintered state and ofremaining solid but shrinking while being subjected to the sinteringtemperature during its transformation from its green state to itssintered state, said platinum material having the property of becomingpartly plastic when subjected to a temperature of the magnitude of saidsintering temperature, and said ceramic carrier and said at least onetubular feedthrough element being secured to each other by a reactionbond established circumferentially around said at least one tubularfeedthrough element during the sintering of said ceramic carrier withsaid at least one tubular feedthrough element received in said at leastone hole with a tight fit, whereby an hermetic seal is developed betweensaid ceramic carrier and said at least one tubular feedthrough elementwithout the application of a braze or other material to the juncturebetween said ceramic carrier and said at least one tubular feedthroughelement.
 2. A feedthrough assembly in accordance with claim 1, whereinone end of said at least one tubular feedthrough element is closed.
 3. Afeedthrough assembly in accordance with claim 1, wherein a face of saidceramic carrier is flat, only one end of said at least one tubularfeedthrough element is sealed, the other end of said at least onetubular feedthrough element is conical in shape, said at least one holein said ceramic carrier is conically flared at that end region of saidat least one hole which intersects said face of said ceramic carrier,and said other end of said at least one tubular feedthrough element isseated in said conically flared end region of said at least one hole insaid ceramic carrier such that said other end of said at least onetubular feedthrough element does not extend past said face of saidceramic carrier.
 4. A feedthrough assembly in accordance with claim 3,wherein said face of said ceramic carrier has a high-quality mirrorsurface finish.
 5. A feedthrough assembly for a prosthetic implant,comprising: a carrier of a sintered ceramic material and having aplurality of holes therein, and a like plurality of 2-ended hollowtubular feedthrough elements of a platinum material each extendingaxially through a respective one of said holes, said ceramic materialhaving a sintering temperature of about 85-90% of the melting point ofsaid platinum material and having the property of being solid both inits green state and its sintered state and of remaining solid butshrinking while being subjected to the sintering temperature during itstransformation from its green state to its sintered state, said platinummaterial having the property of becoming partly plastic when subjectedto a temperature of the magnitude of said sintering temperature, andsaid ceramic carrier and said tubular feedthrough elements being securedto each other by respective reaction bonds established circumferentiallyaround each of said tubular feedthrough elements during the sintering ofsaid ceramic carrier with said tubular feedthrough elements received insaid holes each with a tight fit, whereby respective hermetic seals aredeveloped between said ceramic carrier and said tubular feedthroughelements without the application of a braze or other material to therespective junctures between said ceramic carrier and said tubularfeedthrough elements.
 6. A feedthrough assembly in accordance with claim5, wherein one end of each of said tubular feedthrough elements isclosed.
 7. A feedthrough assembly in accordance with claim 5, wherein aface of said ceramic carrier is flat, only one end of each of saidtubular feedthrough elements is sealed, the other end of each of saidtubular feedthrough elements is conical in shape, each of said holes insaid ceramic carrier is conically flared at that end region of therespective hole which intersects said face of said ceramic carrier, andsaid other end of each of said tubular feedthrough elements is seated insaid conically flared end region of its respective associated hole insaid ceramic carrier such that said other ends of said tubularfeedthrough elements do not extend past said face of said ceramiccarrier.
 8. A feedthrough assembly in accordance with claim 7, whereinsaid face of said ceramic carrier has a high-quality mirror surfacefinish.